LONDON -- The United Kingdom's top cardinal, Keith O'Brien, is to step down as an archbishop, effective immediately, according to a statement published on the website of the Scottish Catholic news office.

Cardinal O'Brien, who is the leader of the Roman Catholic Church in Scotland and the U.K.'s most senior Roman Catholic, also says he is choosing not to take part in the election of the next pope next month to succeed Pope Benedict XVI, a process known as the "conclave.' He said in a statement that he will not attend.

He says he is resigning due to allegations surrounding unspecified inappropriate behavior toward priests in the 1980s. He contests the allegations.

"Approaching the age of seventy-five and at times in indifferent health, I tendered my resignation as Archbishop of Saint Andrews and Edinburgh to Pope Benedict XVI some months ago. I was happy to know that he accepted my resignation 'nunc pro tunc' -- (now -- but to take affect later) on 12 November 2012," said Cardinal O'Brien, in the statement.

"I thank Pope Benedict XVI for his kindness and courtesy to me on my own behalf and on behalf of the people of Scotland, I wish him a long and happy retirement."

It is not yet clear if he is resigning from the College of Cardinals as well. In an interview with the BBC on Friday, Cardinal O'Brien said he found the prospect of choosing the next pope "almost frightening."

Pope Benedict XVI's resignation takes effect on Thursday. And Monday, Benedict issued a letter spelling out line-by-line changes in canon law that will allow cardinals to set an earlier date for electing his successor. The number of eligible electors -- cardinals under the age of 80 -- is now 115.

However, there is growing pressure from U.S. victims of clergy sexual abuse for Cardinal Roger Mahony, the retired Archbishop of Los Angeles, to withdraw from the conclave. More than 30,000 pages of priest personnel records released last month revealed that Mahony had been directly involved in covering up for priests who were known to have abused children and teens.

Mahony has been writing on his personal blog that he plans to attend the conclave and to vote. There's precedent for that. In 2005, Cardinal Bernard Law, who resigned from his post as Archbishop of Boston in 2002 at the height of the explosive abuse scandal, nonetheless voted in the election that chose Benedict.

The last time a cardinal resigned from the College of Cardinals was in 1927, says church historian Matthew Bunson. French Cardinal Louis Billot stepped down because his support for a radical, conservative political movement was in direct conflict with then-Pope Pius XI who opposed it.